The fundamental element of a quantum computer is the quantum bit—also known as the “qubit”. As opposed to a classical bit, representing zero and one, a qubit is also able to represent a quantum superposition of the two states. Hence, the states can be formalized within the laws of quantum physics, with a probability. Accordingly, the states can be manipulated and observed within the laws of quantum physics.
A number of physical objects have been suggested as potential implementations of qubits. However, solid-state circuits, and superconducting circuits in particular, are of great interest as they offer scalability—a possibility of making circuits with a larger number of interacting qubits. Superconducting qubits are typically based on Josephson junctions (JJ). A Josephson junction is basically two superconductors coupled by a weak link. The weak link can for example be a thin insulating barrier, a short section of non-superconducting metal, or a physical constriction that weakens the superconductivity at the point of contact.
A tunable qubit that overcame the known problems related to tuning a qubit, in particular to tuning the Josephson coupling energy using an external magnetic field was disclosed in WO 2016/000836 where a completely different setup that does not require an external magnetic field for tuning the qubit was presented. In particular WO 2016/000836 disclosed a Josephson junction comprising an elongated hybrid nanostructure comprising superconductor and semiconductor materials and a weak link, wherein the weak link is formed by a semiconductor segment of the elongated hybrid nanostructure and wherein the superconductor material has been removed to provide a semiconductor weak link, i.e. a modification of the typical JJ, being a superconductor-insulator-superconductor (SIS) JJs, to a superconductor-normal-superconductor (SNS) JJ, i.e. by replacing the insulator (I) with a normal element (N), where the normal element is a semiconductor material.
The tunable qubit disclosed in WO 2016/000836 was based on a discovery presented in WO 2016/001365 wherein a nanoscale device (or nanometer scale) comprising an elongated crystalline semiconductor nanostructure, such as a nanowire (crystal) or nanowhisker (crystal) or nanorod (crystal), with epitaxial interfaces between the semiconductor and a metal was disclosed. WO 2016/001365 demonstrated the realization of an almost perfect (epitaxial) interface between a semiconductor and a superconductor in the form of a metal, in particular a hybrid nanostructure with InAs and Al. WO 2016/000836 and WO 2016/001365 are incorporated herein in their entirety.
In WO 2016/000836 the Josephson junction was provided ex-situ by standard lithography technique, e.g. etching the superconducting facet layer(s) of a semiconductor nanowire to provide the gap. One problem with ex-situ processing is impurities and the quantum dot formation and uncontrolled electronic environment that can arise as a result hereof. Another common technique for the fabrication of Josephson junctions involves double-angle shadow evaporation of Al through an offset mask, with the tunnel barrier formed by the diffusive oxidation of the Al base layer. However, these shadow masks are typically masking simple substrate surfaces, thereby only providing simple devices and they typically comprise conventional organic bilayers that are incompatible with the ultra-high vacuum (UHV) environment and the high substrate temperatures required for epitaxial film growth. In order to solve the problem of UHV incompatible shadow masks, complex inorganic shadow masks have been developed to allow for in-situ forming of Josephson junctions in simple devices. However, no known methods exist for forming Josephson junctions in more advanced devices.